Marshall Mods - What are your Favourite Mods?

[EL-34/JMP]

First of all we should clarify which "JMP 50" we're speaking of, as the JMP series started in 1967 and comprises a number of different models.
Including the very different 1987 and 2203 circuits.
Marshall did some own mods over the years and even the 2203 could be seen as a modded 1987.

Now as you're familiar with Marshalls and know how to design tone, the question could be: What would you have done to your Marshalls using your present day experience?

Actually a 2203' would more resemble a "modded" 1959' model JMP both being 100 watts.
A JMP 2204' would be closer to a "modded" 1987' both 50 watts. However not all 2204's had cascaded preamps [1977 and after] but the 100 watt 2203's did.
Randall Smith is credited with the first cascaded preamp & master volume about 10 years before any other guitar amp maker.
Frank Levi would be a close second.
Marshall Engineering & Design has always followed someone else's lead.

[Helmholtz]

Actually a 2203' would more resemble a "modded" 1959' model JMP both being 100 watts.
A JMP 2204' would be closer to a "modded" 1987' both 50 watts. However not all 2204's had cascaded preamps [1977 and after] but the 100 watt 2203's did.

You're right, typo. Meant 2204 of course as it's about 50W models.

Randall Smith is credited with the first cascaded preamp & master volume about 10 years before any other guitar amp maker.
Frank Levi would be a close second.
Marshall Engineering & Design has always followed someone else's lead

Maybe partly true, but that's not my point (and not the point of the thread).
Fact is that Marshall did make a number of design changes even in the 60s. All these can be considered mods.


(My favorite Marshall models are the JTM45 and the JTM45/100.
But I like the lead sound with a good 2203, too.)

[Pedro Vecino]

Randall Smith is credited with the first cascaded preamp & master volume about 10 years before any other guitar amp maker.

In 1965?
The two inputs JMPs with master volume are from 1975 if I remember correctly.

[EL-34/JMP]

In 1965?
The two inputs JMPs with master volume are from 1975 if I remember correctly.

Randall Smith was the CEO of MESA / Boogie. He began building HAMM radio's from scratch as a boy scout in the 1950's.
Smith was building & repairing tube guitar amplifiers before Marshall was even a thing.
MESA sold to Gibson a year ago.

[nosaj]

Randall Smith was the CEO of MESA / Boogie. He began building HAMM radio's from scratch as a boy scout in the 1950's.
Smith was building & repairing tube guitar amplifiers before Marshall was even a thing.
MESA sold to Gibson a year ago.

What does it even matter. they all took from someone else. The tube manufactures gave sample circuits on how to use their products.
nosaj

[g1]

Lots of stuff in this thread that has nothing to do with favourite mods. Anybody got anything to offer about specific circuit modifications, like the OP asked about?

[EL-34/JMP]

What does it even matter. they all took from someone else. The tube manufactures gave sample circuits on how to use their products.
nosaj

amp lives matter ... yes we should stop this ridiculous chatter about amplifiers and get back on the topic of amplifiers. What was i thinking.
Does every thread here go so well here or is this just a snafu ?

[Helmholtz]

It's an old thread, last post from the OP was one year ago.

[g1]

yes we should stop this ridiculous chatter about amplifiers and get back on the topic of amplifiers. What was i thinking.
Does every thread here go so well here or is this just a snafu ?[/SIZE][/COLOR][/I]

This is an electronics forum, and this particular discussion is about circuit modifications. Feel free to start a thread about pics of your gear if you would like.

[Chuck H]

First of all we should clarify which "JMP 50" we're speaking of, as the JMP series started in 1967 and comprises a number of different models.
Including the very different 1987 and 2203 circuits.
Marshall did some own mods over the years and even the 2203 could be seen as a modded 1987.

Now as you're familiar with Marshalls and know how to design tone, the question could be: What would you have done to your Marshalls using your present day experience?

Pretty sure it was the 2204 MKIII from memory and looking at the schems. I did make some mods to those amps at a later time but it's hard to recall everything I did. And I was pretty green then so it wouldn't have been my best work. So I'll take this post at face value and consider what I might do today and then re post. But the basic tone was that model pushed with a boost pedal and the master set to just barely clip the power tubes, which seems to shave off the worst of the uglies. With the delay and phase shifter I was able to get useful tones for most stuff in a cover band. I remember adding a chorus pedal at some point when we started covering more blues/rock stuff. Even when I had to turn the master down I could get a good stage tone just clipping the preamp with the boost. Hard to beat a 2204 circuit with a boost pedal.

My early one came with the old components (mustard caps and the big, red carbon comps, etc.) and the later one had the newer box caps and carbon film resistors. I always liked the early one a little better.

[Chuck H]

Ok... If I remember correctly the initial mod made to my JMP's for road use was a change to the bias circuit to run el34 tubes. My earlier one had 6550's when I got it. The later one had el34's in it, but no one ever changed the bias circuit so they were running cold. After the changes to the bias circuits and el34's installed they were biased somewhere just a bit hotter than 70% and I was using those great Tesla el34's available at the time. This is how they were used when I was gigging with them.

More to follow.

[Helmholtz]

So you preferred the EL34s over the 6550s?
How would you describe the difference?

[Chuck H]

So you preferred the EL34s over the 6550s?
How would you describe the difference?

To tell you the truth I was younger and less educated in the lore of tubes. So I swapped them to el34's just because I was drinking the Kool Aid. The only time I heard the 6550's was when I bought the amp so I can't even be sure it was biased correctly. That said, the 6550's had a bit tighter bass and didn't smooth out as much in the HF when driven hard. Since there was no bias adjustment it's not a fair assessment, but the el34's actually did sound better to me. Once upon a time swapping these Marshalls with 6550's to el34's was just what you were supposed to do. I've read that a lot of guys trying for a more metal tone are unmodding these amps to run the 6550's again. Which makes sense from what I heard in my own amp.

[Helmholtz]

I played quite a few JTM Marshalls but they always used EL34s.
A beam tetrode like a 6550 can be expected to perform differently when overdriven.

Did you also change the bias feed resistors to 220k?

[EL-34/JMP]

6550's are not a true "audio" tube, originally designed by General Electric for servo amps in B-52 Bombers.
Marshalls UK JMP amp exports fitted with EL-34's weren't surviving shipping by sea arriving in the USA with busted power tubes.
Marshalls USA distributor Unicord came up with the idea of fitting USA JMP's with 6550's here. [in US].
Ive had both EL-34 & 6550 Marshalls. The 6550's are brutal and don't breakup at all.
50 watt amps w/a pair of 6550's work pretty good but a 100 watt with 6550's is just too much power unless you play Wembley stadium.
In my experience guitar amps always sound better with EL-34's because the midrange just turns to butter the more you crank it.